Semiconductor nanocrystals (NCs) have attracted worldwide scientific and technological interest due to their size-tunable electronic properties (Efros and Efros, 1982; Brus, 1984). These materials are prepared by colloidal procedures, involving surface capping by organic ligands, demonstrating exquisite control of the crystalline quality, size, shape and composition, and the formation of hetero-structures (Spanhel et al., 1987; Peng et al., 1997; Mews et al., 1994; Dabbousi et al., 1997), offering unique flexibility in the design and operation of nano-scaled devices. Efforts to synthesize NCs and in particular colloidal quantum dots (CQDs) thereof, were followed by numerous investigations of the optical and electrical properties of these materials, including exchange interaction, single excitons fine structure (Efros et al., 1996; Francescheti and Zunger, 2000; Chamarro et al., 1996), stimulated emission (Klimov et al., 2000; Choudhury et al., 2005; Lifshitz et al., 2006), multiple excitons generation (Harbold et al., 2005; Beard et al., 2007; Nair and Bawendi, 2007; Nozik, 2008; Trinh et al., 2008), charge injection (Shumway et al., 2001), Auger relaxation (Efros and Rosen, 1996; Klimov et al., 2000), and photoconductivity (Murphy et al., 2006). These phenomena can be a groundwork for the development of NCs-based gain devices (Klimov et al., 2000; Lifshitz et al., 2006), photovoltaic cells (Nozik, 2002; Gunes et al., 2007; Ma et al., 2009; Bang and Kamat, 2009), quantum information light sources (DiVincenzo, 1995), spintronics (Loss and DiVincenzo, 1998; Atatüre et al., 2006) and biological labelling (Dubertret, 2005; Somers et al., 2007). However, they maybe limited by undesired relaxation effects; multiple excitons within CQDs' cores are “squeezed” into a volume comparable to a single bulk exciton, when these cores are surrounded by molecules with low dielectric constant. These conditions induce exciton-exciton attractive Coulomb interactions, which enhance the so-called Auger process (a non-radiative relaxation), leading to a fast quenching (<100 ps) of the multiple excitons' emission (Harbold et al., 2005; Thomas et al., 2006; Beard et al., 2007; Nair and Bawendi, 2007; Mcguire et al., 2008; Nozik, 2008), and to a photoluminescence spectra diffusion and intensity fluctuations (blinking) (Efros and Rosen, 1997; Empedocles et al., 1999). Then, multiple excitons become undetectable in time-integrated experiments, inaccessible for population inversion in gain devices, and avoid multiple carriers' generation in photovoltaic cells. Further on, single exciton, multiple and charged excitons lifetime can be influenced by the existence of surface traps. It is significant to mention that long-lived (˜0.5 ns) multiple excitons do exist in analogous self-assembled quantum dots (SAQDs), which are strained on a semiconductor wetting layer as well as covered by a semiconductor cladding layer (Dekel et al., 2000; Kroner et al., 2008). The differences between the two-quantum dot systems might be related to the dots' volume, dielectric screening of the surrounding, charge neutralization, and carriers' delocalization. This information suggests that persistence of multiple excitons in CQDs will be extended to a timeframe of ˜ns by the suppression of the Auger effect, and it may take place in colloidal hetero-structures, namely core/shell CQDs, and if engineered appropriately, can resemble the SAQDs, without losing the flexibility and applicability inherent in their synthesis.
The IV-VI (e.g., PbSe, PbS) NCs, and in particular the CQDs, are a focus of special interest due to their unique intrinsic properties (Santoni et al., 1992). Bulk PbSe and PbS materials have a cubic (rock salt) crystal structure and a narrow direct band gap (0.28-0.41 eV at 300 K) at the L point of the Brillouin zone. The high dielectric constant (∈∝=18.0-24.0) and the small electron and hole effective mass (<0.1 m*) create an exciton with a relatively large effective Bohr radius (aB(PbSe)=46 nm), eight times larger than that of CdSe. New inter-band optical studies of colloidal PbSe NCs exhibit well-defined band-edge excitonic transitions tuning between 1.0-0.5 eV, and small Stokes shift. In a similar manner, the II-VI NCs and particularly the CQDs are also of a special interest, due to their optical tunnability between the visible and near infrared spectral regime, (with bulk band gap of 1.475 eV, ∈∝=7.4-9.3, aB(CdTe)=7.3 nm), and chemical flexibility at the CQDs' surface.
Various colloidal syntheses have been developed in the last two decades. These colloidal procedures varied mainly by the use of surfactants with different molecular lengths and attraction forces to the NCs surface. Alternatively, core-shell structures consisting of NCs covered by an epitaxial layer of another wide-band semiconductor were formed. Semiconductor nanocrystals that include a core of one or more first semiconductor materials, which may be surrounded by a shell of a second semiconductor material, and are optionally surrounded by a coat of an organic capping agent are disclosed in several US patents granted to Bawendi et al. See, for example, U.S. Pat. Nos. 6,774,361, 6,696,299, 6,617,583, 6,607,829, 6,602,671, 6,576,291, 6,501,091, 6,444,143, 6,426,513, 6,326,144, 6,322,901, 6,319,426, 6,251,303 and 6,207,229, all these patents being hereby incorporated by reference in their entirety as if fully disclosed herein. In U.S. Pat. No. 6,602,671, for example, many semiconductors are mentioned in the description and preferable materials for the core are ZnO, ZnS, ZnSe, ZnTe, CdS, CdO, CdSe, CdTe, MgS, MgSe, GaAs, GaN, GaP, GaSb, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP, InSb, AlAs, AlN, AlP, and AlSb, but the examples show specifically semiconductor nanocrystals in which the core is CdSe and the shell is ZnS.
Fradkin et al. (2003) reported the synthesis and magneto-optical properties of HgTe nanocrystals capped with HgxCd1−xTe(s) alloyed shells.